Many of the industrially important ethylenically unsaturated monomers are highly susceptible to unwanted radical polymerization initiated either thermally or by adventitious impurities. Some examples of these monomers are acrylic and methacrylic acid, acrylate and methacrylate esters, acrylamide and methacrylamide, vinyl acetate and acrylonitrile. Premature polymerization may occur during manufacture, purification or storage of the monomer. Many of these monomers are purified by distillation. It is in this operation where premature polymerization is most likely to occur and to be the most troublesome. Methods to prevent or reduce the amount of such polymerization are thus highly desirable since the prevention or mitigation of such premature polymerization increases the yield of purified monomer and also insures against costly and potentially dangerous runaway polymerization in the plant.
Stable nitroxides are known in the art to be effective in preventing the premature radical polymerization of ethylenically unsaturated monomers. Some examples are seen in Japanese Hei 9-268138 which discloses the stabilization of styrene by 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine and its lower alkyl ethers in the presence of nitrophenols. U.S. Pat. Nos. 3,747,988 describes the use of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine as a polymerization inhibitor for acrylonitrile in the presence of water and oxygen. U.S. Pat. No. 3,488,338 discloses that 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine is an effective chain stopper in the aqueous polymerization of chloroprene. British Patent No. 1,127,127 describes the stabilization of neat acrylic acid by 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine. Japanese Sho 60-36501 describes the stabilization of acrylate and methacrylate esters.
U.S. Pat. Nos. 5,322,960 and 5,504,243 disclose the use of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine in preventing the polymerization of acrylic and methacrylic acids and their esters in the presence of water, but tout the great advantages of using said oxyl compound in combination with manganese acetate, or with hydroquinone and phenothiazine.
EP 178,168 teaches the use of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine or its reaction product with hexamethylene diisocyanate in stabilizing acrylic acid or methacrylic acid in the presence of water.
EP 791,573 discloses that the lower alkyl or aryl esters of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine are effective polymerization inhibitors alone or in combination with various coadditives for vinyl acetate in the presence of water.
Japanese Hei 5-320205 generically describes the use of 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine, its lower alkyl ethers and lower alkanoic esters in preventing the polymerization of acrylic and methacrylic acids alone, but preferably in the presence of chelating agents for ferric salts, such as ethylenediaminetetraacetic acid. The 4-hydroxy, 4-methoxy and 4-acetoxy derivatives are specifically disclosed.
Japanese Hei 5-320217 teaches the stabilization of acrylic and methacrylic acids with 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine, its lower alkyl ethers and lower alkanoic esters alone, but preferably in the presence of phenothiazine, an aromatic amine or phenol. The 4-hydroxy, 4-methoxy and 4-acetoxy derivatives are specifically disclosed.
Since, during the processes to produce and purify various ethylenically unsaturated monomers, water is often present during one of the process steps, there is a long felt need for the stable nitroxide inhibitor to be sufficiently water soluble or miscible to remain homogeneous in wet monomer streams and to prevent polymerization in the aqueous phase and yet for the inhibitor to be able to partition to such an extent that it can prevent polymerization in both the aqeuous phase and in the organic monomer phase for inhibition protection throughout the entire process.